Disclosed in embodiments herein are methods and systems for improving the quality of image rendering in digital printing systems and more particularly to image processing which reassigns tag state values for pixels in image regions affected by a line width control operation.
Recently, image paths for digital printers have been evolving to process contone data rather than binary image data. With the advent of inexpensive digital color printers, methods and systems of digital halftoning have become widely used. It is well understood that most digital printers operate in a binary mode, i.e., for each tonal separation, a corresponding spot is either printed or not printed at each specified location in a printed image. Digital halftoning controls the printing of tonal spots, where spatially averaging the printed spots by the human visual systems of one or more tonal separations provides the illusion of the required continuous tone (contone) gray levels.
Anti-aliasing in the context of digitizing line art and certain graphical image structures is a known method of using intermediate levels of intensity to achieve subpixel position of edges for several reasons including reduction or elimination of jaggies on the edges of lines and polygons, including text. Jaggies are typically most visible at the edges of sloped lines approaching horizontal or vertical. Anti-aliased images can be generated by capturing the image at a resolution greater than the final or desired output resolution, then reducing the resolution of the image by sub-sampling using an averaging process. A major benefit of anti-aliased images is that high contrast, saturated objects are surrounded with pixels possessing intermediate values that visually suggest the true, higher resolution position of object edges.
To optimally render anti-aliased pixels, it is beneficial to recognize pixels as anti-aliased, or not anti-aliased, and render them accordingly with specific rendering techniques. Metadata in the form of a tag, or tag state for each pixel can be used to enable such recognition and appropriate rendering. Pixel tag states can also be used to provide other information for specialized rendering techniques as well.
The field of digital printing has also employed numerous methods for line width modification in digital images. Some methods have been developed to correct for marking process shortcomings, such as inability to print narrow black lines, as taught by U.S. Pat. No. 4,791,679, entitled “Image Character Enhancement using a Stroke Strengthening Kernel”, to Barski and Gaborski, U.S. Pat. No. 4,544,264, entitled “Fine Line Enhancement”, to Bassetti, and U.S. Pat. No. 4,625,222, entitled “Interacting Print Enhancement Techniques”, or to print narrow white lines as taught by U.S. Pat. No. 6,944,341 entitled “Loose Gray-Scale Template Matching For Image Processing Of Anti-Aliased Lines”, to Loce, et at., among others. Other methods have been developed to achieve a certain darkness or lightness appearance, and may be referred to as “appearance tuning” or “appearance matching”, such as those taught by U.S. Pat. No. 6,606,420 entitled “Method and apparatus for digital image darkness control in saturated image structures”, to Loce, et al., U.S. Pat. No. 6,181,438, entitled “Method and Apparatus for Digital Image Darkness Control Using Quantized Fractional Pixels”, to Bracco, et al., and U.S. Pat. No. 5,128,698 entitled “Boldness Control in an Electrophotographic Machine”, to Crawford et al.
It has been found, however, that these line width modification operations can result in inaccurate or irrelevant pixel tag state information which can lead to image quality defects in subsequent rendering operations. The present systems and methods described herein address these problems and others providing improved image rendering.